Iridescence is when surfaces – such as the wings of butterflies – appear to change brightness and colour as the angle of view, or the angle of illumination, changes. The iridescent wings of Morpho butterflies are some of the best examples. The spectral reflectance changes with the angle of incidence – i.e. the viewing angle – relative to the sun, and can change colour, e.g. from blue-green to blue, as the butterfly flaps its wings.

One very common example of iridescence is a soap bubble! When light shines onto a bubble it changes colour as a result of the interference of light reflecting off the front and back surfaces of the thin soap film (below).

Similarly, on butterfly wings, diffracted light waves interfere with each other so that certain colour wavelengths cancel out (destructive interference) while others are intensified and reflected (constructive interference). Tiny nano-structures on the surfaces of the butterfly wing scales (below) cause these effects.

The lower lamina of both the cover and ground scales acts as an optical thin film reflector in the Morpho genus, producing  the blue structural coloration of the wings (Giraldo & Stavenga, 2016).

Iridescence is usually only visible within a narrow range. Since the colours are brightest from certain directions and body orientations, they can be employed to direct a visual signal to particular receivers. Quite often, the signaller is a male trying to show off his flashy colours; either to intimidate the opposition, or impress a female.

The elytra of many beetles (Coleoptera) are highly iridescent, especially those of scarab beetles, or chrysomelids like the the rainbow leaf beetle (below). Iridescent surfaces change in colour with viewing angle – so the colours are highly directional.

Damselfly males like the Beautiful Demoiselle (below) try catch the attention of females passing through their territories using displays which show off their strikingly coloured wings and bodies. The bright wings are probably also used to intimidate other males during territorial fights.

The dark-blue wings of mature male Calopteryx damselflies (below) are due to darkly coloured wing membranes and blue reflecting veins (Stavenga et al., 2012). These multilayered wing veins act as interference reflectors, and the the blue reflections are thought to be due to nano-structures: distinct multilayers, with a period about 100 nm, parallel with the wing vein surface (Stavenga et al., 2012).

In butterflies, elaborate three-dimensional nano-structures – so-called perforated multilayers – between the lamellae of the wing scales, also cause cause iridescent colours, as I discussed in my book Courtship and Mating in Butterflies (Cannon, 2019). Like the lovely little lycaenid butterfly shown below.

Iridescent wing scales on butterfly wings are often very striking and apparent in flight. Flapping the wings causes changes in the brightness and colour as the angle of view, or the angle of illumination changes. This conspicuous flashing is called flicker contrast (below).

The brilliant violet-blue iridescent colours produced on the dorsal wing surfaces of male emperor butterflies, like the European Purple emperor (Apatura iris), or the Indian purple emperor (Mimathyma ambica) are only visible within a very narrow (ca. 18°) angular range (below and top).

Iridescence is rather a complex science! For example, there are three main types or classes of iridescence in beetles: multilayer reflectors, three-dimensional photonic crystals and diffraction gratings (Seago et al., 2009). The beetle show below has, I think, a ‘metallic iridescence’.

It used to be thought that these metallic green iridescent colours on scarab beetles were aposematic, warning colours, advertising their unpalatability to bird predators (Vulinec, 1997). But, it is much more likely that they are used in mating, perhaps advertising the shiny brilliance of the male for the female to assess, choose or reject!

There are dull versions of this species, Trypocopris pyrenaeus, but I have never been able to work out if they are male or female! There are also different hues of metallic colours, but bear in  mind that these colours can change with the angle and the quality of the light!

There are some wonderfully bright and colourful iridescent carabid beetles (Carabidae) caused by ‘engraved microlines, or of very dense transverse lines, producing a system of diffraction gratings causing iridescence’ (Lindroth, 1974). The one shown below seems to have bagged an earthworm!

Here’s another shiny carabid I photographed in Spain (below) showing off its diffraction gratings! There is a theory that the diffraction gratings – the microsculpturing – on the exterior cuticle of the outer surfaces of iridescent beetle wing cases help them to slide through leaf litter or soil (Wei et al., 2019).

Finally, many other insects show iridescent colours, including wasps, flies and many others. Can’t wait to find some more to photograph!

References

Cannon, R. J. (2019). Courtship and Mating in Butterflies. CABI. (https://books.google.co.uk/books/about/Courtship_and_Mating_in_Butterflies.html?id=XwHyxgEACAAJ&redir_esc=y)

Giraldo, M. A., & Stavenga, D. G. (2016). Brilliant iridescence of Morpho butterfly wing scales is due to both a thin film lower lamina and a multilayered upper lamina. Journal of Comparative Physiology A, 202(5), 381-388.

Lindroth, C. H. (1974). On the elytral microsculpture of carabid beetles (Col. Carabidae). Insect Systematics & Evolution, 5(3-4), 251-264.

Stavenga, D. G., Leertouwer, H. L., Hariyama, T., De Raedt, H. A., & Wilts, B. D. (2012). Sexual dichromatism of the damselfly Calopteryx japonica caused by a melanin-chitin multilayer in the male wing veins. PloS one, 7(11), e49743.

Thomas, D. B., Seago, A., & Robacker, D. C. (2007). Reflections on golden scarabs. American Entomologist, 53(4), 224-230.

Vulinec, K. (1997). Iridescent dung beetles: a different angle. Florida entomologist, 132-141.

Wei, L., Reiter, K. E., McElrath, T., Alleyne, M., & Dunn, A. C. (2019). Diffraction gratings alter the surface friction of iridescent beetle cuticle against fibrous surfaces. Biotribology, 20, 100108.